Limiting Variable Scope Using (function(){})();

Simply put, a scope limiter is a self-executing function that defines a variables, performs a task, and clears those variables so that their scope is limited to the function itself. Take the following JavaScript code, for example:

Everything above works but the second task can "see" the variables used to complete task one. This is undesirable as those variables from the first task could cause problems with later tasks. The better way to complete the two tasks is to use scope limiters for each:

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In this case you could define $$('a') globally but my example was more trying to display the two closures working separately.

Will

Doesn’t this sort of miss the point of closures? I always thought that ‘closure’ meant it ‘closed’ over variables available to it when being declared, so those variables were available even when they went out of scope. It’s hard to explain, but this example might help:

So, you’d call makeClosure, which would return a function. Whenever the returned function was called, the value of t (“foo”) would still be available to it (thus returning “foobar”) even though t’s scope had expired when makeClosure ended.

TheBigBabou

Sorry David, I don’t want to be offensive, but your code examples are no closures. What you are showing and describing is limiting variables to function scope.

A closure is a function or codepointer which accesses variables outside its scope. Like here:

The closure here would be the function, which is the first parameter to setTimeout. It doesn’t declare a variable “out”, but is able to access “out” from its outer function scope. As long as a variable stores a reference to this function, “out” is not freed from memory as well.